Adding an additional bathroom

First - go get a permit. Ask the building Dept what they see as issues or problems as they will be the ones to make this a go or no-go.

Biggest issues are usually getting to the drain line. If you're on a slab foundation, this means jack-hammering up the concrete. If on a raised foundation, it's MUCH easier.....

You will also need a separate (usually)... 20A circuit for the GFCI. Sometimes you can splice into the other bathroom circuit, depending on how many outlets it serves....

Other than that, it's just basic construction stuff. Run your numbers....The cost will be around $20k for a basic bathroom, is there enough ROI to make that work?